1. Field of the Invention
The present invention relates to a method and an apparatus for producing a conductive material. For example, the conductive material is suitable for use in electromagnetic shielding materials for blocking electromagnetic waves from display devices such as PDPs (plasma display panels) and liquid crystal displays, transparent electrodes of electronic devices such as touch panels, printed circuits of electronic devices, antenna circuits of contactless ICs, etc.
2. Description of the Related Art
A conductive material such as an electromagnetic shielding film for PDP is required to have both conductivity and light transmittability. A material having a transparent substrate and a mesh-patterned conductive layer of a fine metal wire, etc. has been known as such a conductive material, and it can be produced by (1) a method containing the steps of forming a thin copper layer on a transparent substrate by bonding, electroless plating, etc., and subjecting the layer to a photolithography process to etch the layer into a pattern (Japanese Laid-Open Patent Publication Nos. 05-016281 and 10-163673, etc.), (2) a method containing the steps of printing an ink containing an electroless plating catalyst particle such as a palladium particle into a pattern on a transparent substrate, and subjecting the printed ink to an electroless plating treatment to form a conductive layer thereon (Japanese Laid-Open Patent Publication Nos. 11-170420 and 2003-318593, etc.), or (3) a method containing the steps of exposing a photosensitive silver halide layer formed on a transparent substrate to form a patterned developed silver, and subjecting the developed silver to a plating treatment to form a patterned conductive layer thereon (International Patent Publication No. WO 01/51276, Japanese Laid-Open Patent Publication No. 2004-221564, etc.)
The method of (1), which contains etching and patterning the thin copper layer by the photolithography process, is advantageously capable of micromachining. Thus, by using the method, a mesh having a high aperture ratio can be formed, and a conductive material having high conductivity and transparency can be obtained easily. However, this method disadvantageously requires complicated processes of formation of the thin copper layer on the substrate, formation of a photosensitive resin layer on the thin copper layer, exposure, removal of the resin layer, removal of the thin copper layer by etching, etc. Further, most of the formed thin copper layer must be removed, thereby resulting in increased cost for waste liquid treatment.
The method of (2), which contains printing the plating catalyst into a pattern and subjecting the printed catalyst to the plating treatment, is disadvantageous in that the printed wire cannot be formed with a small width easily. Thus, the conductive layer often has an excessively large width, causing deterioration of the light transmittance and image quality (e.g. moire generation) in a display device. Furthermore, because a particle of the expensive palladium or another metal having a lower activity (such as copper or silver) is used as the electroless plating catalyst, this method has a disadvantage of high cost or low productivity.
The method of (3), which uses the silver halide, contains simpler processes as compared with the photolithography method, and can form the fine wire more easily as compared with the printing method. Furthermore, this method is suitable for forming a seamless continuous conductive layer. However, the developed silver exhibits a low activity in the electroplating or electroless plating treatment, so that the plating time is increased. Thus, this method has a disadvantage of low productivity.
In a technique for increasing the electroless plating activity of the developed silver described in Japanese Laid-Open Patent Publication No. 2004-221564, the developed silver is treated with a palladium-containing solution to increase the electroless plating rate. However, in this technique, the expensive palladium catalyst is needed, and so-called fog silver, which is developed silver unexpectedly generated in a portion other than the conductive layer, is activated by the palladium, whereby a plated layer is undesirably formed in the portion, causing a so-called plating fog. In a technique described in Japanese Laid-Open Patent Publication No. 2007-012314, the thiosulfate salt concentration of a fixer is controlled, whereby the surface resistance is lowered after the electroless plating, to reduce the plating time. However, in this technique, palladium chloride is used as a catalyst, and the plating rate is insufficient. In a technique described in Japanese Laid-Open Patent Publication No. 2006-228836, the electroless plating activity of the developed silver is improved without palladium catalysts by immersing in an activation solution containing sodium borohydride, silver nitrate, etc. However, this technique cannot be practically used because of the low stability and high cost of the activation solution.
In a technique for increasing the electroless plating rate of the developed silver or another component, as disclosed in Japanese Laid-Open Patent Publication Nos. 61-034180, 02-175895, and 2004-018975, etc., a current or a negative electric potential is applied using the material to be plated as a negative electrode in an electroless plating solution. However, in this technique, the plating metal is disadvantageously deposited also on a counter positive electrode placed in the electroless plating solution. Though the plating on the positive electrode can be prevented by controlling the timing of taking it out from an electroless plating bath, putting it in the electroless plating bath, or applying the current, this technique requires complicated operation and is unsuitable for continuous mass production of the conductive film.
Also electroplating of the developed silver, not the electroless plating, has been tested. However, in this technique, as described in Japanese Laid-Open Patent Publication No. 2007-009326, etc., the developed silver has a low conductivity, so that the plated layer is ununiformly formed only in the vicinity of an electrode and a low-conductive portion. Further, the applied voltage or current must be controlled to prevent wire breaking due to Joule heat, whereby the metal is generally deposited only in a limited amount. To increase the plating metal amount, the electroplating has to be carried out at a low rate under a controlled current, or using a multistage apparatus as described in this patent publication. Therefore, this technique cannot be practically used. This problem is caused not only in the method using the developed silver, but also in the method of printing the fine metal particle into a pattern or the like. Thus, in a case where the fine wire is formed from the dispersion of conductive fine particles of a metal, etc. and subjected to an electroplating treatment, the current for electroplating is limited because of the contact resistance between the conductive fine particles and the insufficient conductivity of the fine wire. Thus, the electroplating has to be carried out at a low rate or in a multistage apparatus to increase the plating amount. When the density or application amount of the conductive fine particles is increased to improve the conductivity, the wire width is increased and the adhesion between the conductive layer and the substrate is deteriorated. When the electric resistance between the conductive fine particles is lowered beforehand by sintering, etc., a complicated operation is required, and the substrate is often thermally deformed.
Accordingly, there is a demand for a conductive metal layer producing method with high productivity, capable of forming a fine wire easily.